In drilling well bores it is often very difficult and costly to maintain the course of the hole or well bore within acceptable limits. Deviation refers to the angle which the well bore departs from the desired course. Although not always the case, most well drilling attempts to achieve a near vertical hole axis or course. Deviation may be caused by many factors, including tilted geological formations which force the bit to deviate in the direction of the tilt, bending of the drill string due to the weight on it, and other influences from the side wall of the well bore.
In those geographic regions where such tilted geological formations are prevalent, many hours or days are spent rotating the bit with substantially reduced weight on it, because reducing the weight on the bit can decrease those influences which tend to increase deviation. Under substantially reduced weight conditions, hole penetration rates are often reduced to one to five feet per hour where normal weight conditions on the bit would achieve 20 to 50 feet per hour. Of course, reducing the penetration rates increases the cost of drilling the well bore, because more hours of rig time will be spent in achieving the depth ultimately desired.
There are sophisticated methods for correcting deviation, but these techniques are seldom economical in drilling wells which are considered marginal producing prospects. Down hole mud motors are very effective in reducing deviation, but such down hole motors are too expensive for use in marginal producing prospects. Whipstocks and multiple stabilizers can also be uneconomical. Most of these correctional techniques require the drilling to be halted, the drill string withdrawn from the well bore by disconnecting the drill pipes one at a time, installing the special correctional equipment, and then reassembling the drill pipes into the drill string. This drill string removal and reinsertion process, known as "tripping", consumes additional time, results in no productive drilling and increases the cost of drilling. Very often another "trip" is required to remove the special correctional equipment after the deviation has been corrected.